It is known that atmospheric air contains components that have to be eliminated before the said air can be let into the heat exchangers of the cold box of an air separation unit, these in particular being the carbon dioxide (CO2), water vapour (H2O), oxides of nitrogen and/or hydrocarbon compounds for example.
This is necessary because if the air is not pretreated in this way to eliminate its CO2 and water impurities these impurities will solidify into ice during the cooling of the air to cryogenic temperatures typically of −150° C. or below, and this may result in problems with the equipment, particularly the heat exchangers, the distillation columns, etc. becoming plugged.
Furthermore, it is also commonplace to at least partially eliminate the hydrocarbons and oxides of nitrogen impurities likely to be present in the air in order to prevent them from becoming too highly concentrated in the bottom of the distillation column or columns and thus guard against any risk of equipment damage.
At the present time, this air pretreatment is performed, according to circumstance, using the TSA (temperature swing adsorption) method or using the PSA (pressure swing adsorption) method; what PSA method means is actual PSA methods proper, VSA (vacuum swing adsorption) methods, VPSA methods and the like.
The invention applies to the various methods and units that use radial adsorbers, operating in TSA mode, that is to say with an increase in temperature during the regeneration.
Conventionally, an air purification TSA method cycle involves the following steps:    a) purifying the air by adsorbing impurities at superatmospheric pressure and at ambient temperature,    b) depressurizing the adsorber down to atmospheric pressure,    c) regenerating the adsorbent at atmospheric pressure, particularly using the residual gases, typically impure nitrogen from an air separation unit and heated to a temperature usually of between 100 and 250° C. by means of one or more heat exchangers,    d) cooling the adsorbent to ambient temperature, particularly by continuing to let into it the said residual gas from the air separation unit, but not heated this time,    e) repressurizing the adsorber with purified air from, for example, another adsorber that is in the production phase.
In general, the air pretreatment devices comprise two adsorbers, operating in alternation, that is to say that one of the adsorbers is in the production phase while the other is in the regeneration phase.
The production phase corresponds to the purification of the gaseous mixture by adsorption of the impurities.
The regeneration phase corresponds to the desorption of the impurities held on the adsorbent during the adsorption step, by heating the adsorbent using the residual gases heated to a temperature of between 100° C. and 250° C. It involves the steps of depressurizing, heating, cooling and repressurizing.
A step of paralleling the two adsorbers, of varying duration, that is to say lasting for a few seconds to several minutes, is generally added to the start or end of the regeneration phase.
Such TSA methods for purifying air are described in particular in documents U.S. Pat. No. 3,738,084 and FR-A-7725845.
As soon as the flow rates to be purified become high, it is known practice to use radial adsorbers as taught by document U.S. Pat. No. 4,541,851 or in patent EP-A-1,638,669.
Radial adsorbers allow large quantities of fluid, particularly of atmospheric air, to be purified by adsorption reliably and repeatedly while at the same time maintaining a good distribution of the treated fluid and fluid flow velocities that are compatible with the mechanical properties of the adsorbent particles used.